Processes for the preparation of stable, aqueous plastics dispersions, for example, polyurethane polyurea dispersions, are already known, e.g., German Pat. Nos. 1,182,946 and 1,178,586; German Auslegeschrift No. 1,237,306; German Offenlegungsschriften No. 1,495,745; 1,595,602; 1,770,068; 2,019,324 and 2,314,512; U.S. Pat. Nos. 3,756,992; 3,686,108 and 3,905,929 and D. Dieterich et al, Angew. Chem. 82, 53 (1970). The dispersions described in these references are based on the principle of introducing hydrophilic centers into a macromolecular chain of a polyurethane (polyurea) molecule. In the known dispersions, these hydrophilic centers or so-called internal emulsifiers are ionic groups or ether functions. They are either built into the prepolymer in the form of special diols or used as modified amines for chain lengthening the prepolymers which invariably have at least two isocyanate end groups.
Various processes are available for preparing the known dispersions and have been described, for example, in D. Dieterich and H. Reiff, Angew. Makromol. Chemie 26, 85 (1972). As a general rule, the solution of a polyurethane in an organic solvent is either converted into an aqueous dispersion or a prepolymer in the form of a liquid is dispersed in water, with or without solvent. For example, a liquid prepolymer ionomer containing isocyanate groups may be introduced into water with vigorous stirring; an emulsion of the prepolymer is initially formed, which then continues to react with water or a diamine or polyamine to undergo chain lengthening to a high molecular weight polyurethane urea.
One particularly simple method of dispersion has been described in German Offenlegungsschrift No. 1,913,271 or U.S. Pat. No. 3,756,992. According to this method, a solid or liquid polyurethane polyelectrolyte which is capable of undergoing addition with formaldehyde to form methylol groups is dispersed by mixing with water and converted into a polyurethane polyelectrolyte containing methylol groups by addition of formaldehyde or formaldehyde derivatives. This polyurethane polyelectrolyte is then condensed to the high molecular weight polyurethane either in the dispersion or on a substrate.
To effect dispersion, water is added to the stirrable melt until it forms the continuous phase, the system in most cases first passing through the water-in-oil emulsion stage. Alternatively, a water-in-oil emulsion may be prepared at an elevated temperature, this emulsion then changing into an oil-in-water emulsion on cooling. Preparation of the dispersion is practically always carried out at elevated temperatures, preferably at between 50.degree. C. and 120.degree. C. This is necessary, first because the prepolymer stage which is to be dispersed is too highly viscous at room temperature and would, therefore, require elaborate apparatus such as screw extruders for dispersion at that temperature, and second because the rate at which dispersion takes place generally increases with the temperature employed. Dispersion of solvent-free melts of isocyanate prepolymers is also almost always carried out at elevated temperatures.
Although the procedure described above is economically very advantageous compared with the preparation of dispersions from solutions and can be carried out with simple apparatus, it is not satisfactory for all practical requirements. Non-ionic dispersions are very difficult or even impossible to prepare by this method because the dispersability of polyurethane precursors which contain hydrophilic polyether groups diminishes with increasing temperature. This means that for dispersion at the elevated temperatures required, it is necessary to increase the hydrophilic character of the compound to a greater extent than is necessary for the subsequent stability of the dispersion. The water resistance of the product is thereby reduced. On the other hand, the preparation of polyurethanes which have been modified with hydrophilic ether groups would be particularly desirable because dispersions of these polyurethanes have a high resistance to frost.
The usual ionomer dispersions generally have insufficient resistance to frost, so that storage and transport during the cold season of the year are expensive because they require heated storage rooms. Moreover, the transport of aqueous dispersions over long distances is in itself unsatisfactory because of the added expense of transporting large quantities of water.
There is, therefore, a demand for the processer to be able to prepare his own dispersions from the solid materials and water in much the same way that organic solutions are generally prepared.
One step in this direction is the supply of redispersible powders, which are already known. The manufacture of these powders is technically complicated and undesirable on economic grounds because the powders are prepared from a previously prepared dispersion by an expensive freeze drying or spray drying process, as described in German Auslegeschrift No. 1,729,201. It would, therefore, be desirable to have available a process for the production of a solid material which is stable in storage and which could be converted into an aqueous dispersion at a later date without the complicated intermediate stage of an aqueous primary dispersion.
In that way, the full economic advantage would be gained from applying polymers from the aqueous phase.
The solution to this problem has generally been regarded as basically impossible because dispersions, being metastable diphasic systems, cannot be obtained spontaneously by a process of solution but, on the contrary, tend to deposit solids irreversibly under the influence of chemical or physical changes, which is the reason why stabilization of the dispersions plays a very important commercial role. It is well known to the man of the art that coatings which have been deposited from dispersions cannot be reconverted into the dispersions by the action of water. The commercial applications of dispersions, in fact, depend on the impossibility of direct redispersion. Very hydrophilic dispersion coatings are exceptions to this rule, but, in view of their lack of resistance to water, they are of no commercial importance.